Boron nitride agglomerated powder

ABSTRACT

Novel boron nitride agglomerated powders are provided having controlled density and fracture strength features. In addition methods for producing same are provided. One method calls for providing a feedstock powder including boron nitride agglomerates, and heat treating the feedstock powder to form a heat treated boron nitride agglomerated powder. In one embodiment the feedstock powder has a controlled crystal size. In another, the feedstock powder is derived from a bulk source.

BACKGROUND

1. Field of the Invention

The present invention relates generally to methods for producingagglomerated boron nitride powders, powders formed thereby, andcomponents incorporating such powders.

2. Description of the Related Art

Microelectronic devices, such as integrated circuit chips, are becomingsmaller and more powerful. The current trend is to produce integratedchips that are steadily increasing in density and perform more functionsin a given period of time over predecessor chips. This results in anincrease in power consumption and generation of more heat, andaccordingly, heat management has become a primary concern in thedevelopment of electronic devices.

Typically, heat generating sources or devices, such as integratedcircuit chips, are mated with heat sinks to remove heat that isgenerated during operation. However, thermal contact resistance betweenthe source or device and the heat sink limits the effective heatremoving capability of the heat sink. During assembly, it is common toapply a layer of thermally conductive grease, typically a siliconegrease, or a layer of a thermally conductive organic wax to aid increating a low thermal resistance path between the opposed matingsurfaces of the heat source and the heat sink. Other thermallyconductive materials are based upon the use of a binder, preferably aresin binder, such as, a silicone, a thermoplastic rubber, a urethane,an acrylic, or an epoxy, into which one or more thermally conductivefillers are distributed.

Typically, these fillers are one of two major types: thermallyconductive and electrically insulative, or thermally conductive andelectrically conductive fillers. Aluminum oxide, magnesium oxide, zincoxide, aluminum nitride, and boron nitride are the most often citedtypes of thermally conductive and electrically insulative fillers usedin thermal products. Boron nitride is especially useful in that it hasexcellent heat transfer characteristics and is relatively inexpensive.

However, in order to achieve sufficient thermal conductivity withpresently used fillers, such as boron nitride, it has been necessary toemploy high loadings of filler in the binder. See, for example, U.S.Pat. Nos. 5,898,009, 6,048,511, and European Patent No. EP 0 939 066 A1,all to Shaffer et al., which teach an alternate methodology to achievesolids hexagonal boron nitride loading approaching 45 vol. %.

There continues to be a need for improved thermally conductive fillermaterials and methods for forming such materials. In particular, methodsare needed by which such materials can be produced economically and inlarge volumes, with improved control over properties of the finalproducts. In addition, there continues to be a need for improved boronnitride powders, including controlled density powders such as low andmedium density powders that maintain sufficient strength for handlingand deployment in applications such as in the semiconductor area.

Beyond use of boron nitride powders as a filler material for thermalconductivity applications, there is also a need in the art to produceboron nitride powder having desired and targeted properties fordeployment in other end-uses, such as in friction-reducing applications.In this regard, a need exists for highly flexible fabrication processes,which can be used to produce boron nitride powders having widely varyingphysical, thermal, electrical, mechanical, and chemical properties withhigh yield and using cost-effective techniques.

SUMMARY

According to one aspect of the present invention, a boron nitrideagglomerated powder has an agglomerate fracture strength to tap densityratio of not less than about 11 MPa·cc/g,

According to another aspect of the present invention, a boron nitrideagglomerated powder an agglomerate fracture strength to envelope densityratio of not less than 6.5 MPa·cc/g.

According to one aspect of the present invention, a method is providedfor forming a boron nitride agglomerated powder, in which a feedstockpowder is utilized that contains boron nitride agglomerates. Thefeedstock powder generally has fine crystals having a particle size notgreater than about 5 μm. The feedstock powder is then heat-treated, toform a heat-treated boron nitride agglomerated powder.

According to another aspect of the present invention, a microelectronicdevice is provided including an active component, a substrate, and athermal interface material provided between the active component and thesubstrate. The active component typically generates heat, and thethermal interface material includes an agglomerate having a fracturestrength to envelope density ratio not less than 6.5 MPa·cc/g.

According to another aspect of the present invention, a printed circuitboard is provided, including multiple layers having at least one layercomprising agglomerates having a fracture strength to envelope densityratio not less than 6.5 MPa·cc/g.

According to yet another feature of the present invention, a compositestructural component is provided including a matrix phase andagglomerates having a fracture strength to envelope density ratio notless that about 6.5 MPa·cc/g.

According to another aspect of the present invention, a method forforming a boron nitride agglomerated powder is provided, in which a bulkboron nitride powder containing agglomerates is provided. Then, aportion of the boron nitride agglomerates is removed from the bulkpowder, to form a feedstock powder, and feedstock powder is heated toform a boron nitride agglomerated powder.

According to another embodiment of the present invention, a method forforming a boron nitride agglomerated powder is provided, in which afeedstock powder is provided which includes boron nitride agglomeratescontaining turbostratic boron nitride. The feedstock powder is thenheat-treated to form heat-treated boron nitride agglomerated powder.

According to a feature of the present invention, the boron nitrideagglomerated powder following heat treatment may be subjected to amechanical agitating operation, such as crushing. This process iseffective to break weak inter-agglomerate bonds which typically formduring the heat treatment process, such that the particle sizedistribution resembles or closely approximates that of the originalfeedstock powder. Typically, at least 25 wt. % of the heat-treated boronnitride powder following crushing falls within an initial particle sizerange of the feedstock powder.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention may be better understood, and its numerousobjects, features, and advantages made apparent to those skilled in theart by referencing the accompanying drawings.

FIG. 1 is a flow chart showing a particular process flow for forming aboron nitride agglomerated powder according to an embodiment of thepresent invention.

FIG. 2 illustrates the ideal crystal structure of hexagonal boronnitride.

FIG. 3 illustrates a testing apparatus used to characterize embodimentsof the present invention.

FIG. 4 illustrates a cross sectional view of a printed circuit boardaccording to an embodiment of the present invention.

FIG. 5 illustrates a microelectronic device including an integratedcircuit bonded to a substrate through use of a thermal transfer film.

FIG. 6 illustrates a laptop computer incorporating a case according toan embodiment of the present invention.

The use of the same reference symbols in different drawings indicatessimilar or identical items.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

Turning to FIG. 1, a general process flow for forming an agglomeratedboron nitride powder is illustrated. The process flow begins withprovision of a boron nitride briquette or pellet. Typically, the boronnitride briquette or pellet is formed of boron nitride powder that ispressed together in the form of a briquette or pellet. The size of thebriquette or pellet is not particularly important, and it is density canvary widely depending upon the process (e.g., filter cake, roll compact,pill press, isostatic press) to create the boron nitride briquette orpellet. While embodiments of the present invention take advantage ofrelatively small briquettes or pellets such as on the order of a fewgrams, larger briquettes such as on the order of 100 kg may also beprocessed.

A goal of the initial processing of boron nitride briquette or pellet isto provide a feedstock powder that is used according to embodiments ofthe present invention. The feedstock powder is generally formed by firstcrushing the boron nitride briquette or pellet at step 10. Suitablemethods for crushing the briquette include jaw crushing and rollcrushing. The briquette or pellet is crushed into agglomerates of boronnitride having a desired agglomerate size or diameter. Preferably, thebriquette or pellet is crushed into agglomerates of boron nitrideranging from about 10 microns to about 1,000 microns. In addition to jawcrushing and/or roll crushing, the bulk powder may be milled, so as toform even smaller particles, such as particles formed of very finecrystals, such as crystals less than 10 microns in size.

In one embodiment, following crushing to form the bulk powder utilizingany combination of jaw crushing, roll crushing and/or fine milling, thebulk powder is classified at step 16 to form a desired feedstock powder,for later processing. Coarse agglomerates that are greater than thetarget particle size may be re-crushed and classified until they arewithin the target size distribution. However, it is generally moretypical to press the bulk powder at step 12. Typically, pressing iscarried out in the form of cold pressing or isostatic pressing to form anew log, briquette, or pellet at this intermediate step, which hasdesirable crystalline and B₂O₃ content properties. Following pressing,the new log, briquette, or pellet is crushed at step 14. The pressingand crushing steps 12 and 14 may be repeated any number of times tomodify the crystal size, particle size, particle size distribution, andB₂O₃ content of the resulting feedstock powder.

The feedstock powder as well as the bulk powder that is classified atstep 16 contain agglomerates. As used herein, an agglomerate is acollection of boron nitride crystals that are bonded together to form anindividually identifiable particle. Although such agglomerates aretypically formed of crystals, the agglomerate may be partially or fullyglassy, such as in the case of impure or turbostratic boron nitride.

In accordance with an embodiment of the present invention,non-agglomerated boron nitride particles (e.g., platelets or crystallinedomains) are removed from the powder, as well as the agglomerates notwithin the desired feedstock powder particle size distribution. Suchnon-agglomerated boron nitride particles are typically less than 10microns in size. Preferably, non-agglomerated boron nitride particlesare removed to less than about 5%, more preferably, to less than about1%, such as less than about 0.1%. Suitable techniques for removing thenon-agglomerated particles include screening, air classifying, andelutriation, (see Chem. Eng. Handbook, Perry & Chilton, 5^(th) Ed.,McGraw-Hill (1973), which is hereby incorporated by reference in itsentirety). As such removal methods are well known in the art, they willonly be discussed briefly herein.

Typically, classification is carried out by screening. Screening is theseparation of a mixture of various sized solid particles/agglomeratesinto two or more portions by means of a screening surface. The screeningsurface has openings through which the smaller particles/agglomerateswill flow, while the larger particles/agglomerates remain on top. Thisprocess can be repeated for both the coarse and smallparticle/agglomerate size streams, as many times as necessary, throughvarying screen openings to obtain a classification ofparticles/agglomerates into a desired particle/agglomerate size range.

According to one feature of the process flow shown on FIG. 1, aftercrushing and classification, as well as with the optional pressing andcrushing operations, steps 12 and 14, a feedstock powder is providedthat has desirable properties. The feedstock powder is formed ofagglomerates within a particular, predetermined particle size range thatis taken from or removed from the bulk boron nitride agglomerate powder.Here, the individual agglomerates are composed of fine crystals, alsoknown as crystalline domains. These crystals are bonded together viaintra-agglomerate bonding and are individually identifiable through SEManalysis. Generally, it is desired that the crystals have an averagecrystal size no greater than about 5 microns. More preferably, thecrystals have an average crystal size no greater than about 4 microns,even more preferably no greater than about 2 microns.

While the foregoing initial processing to form a feedstock powder hasbeen described in connection with crushing a boron nitride briquette, itis understood that the feedstock powder may be prepared utilizingdifferent processing techniques e.g., carbothermic reduction of boricoxide in the presence of nitrogen, reaction of melamine compounds,acting as a source of strongly reducing nitrogen compounds, to reduceboric oxide to boron nitride and direct nitridation of boric oxide byammonia. Alternatively, reagents such as boron trichloride and ammoniacan be reacted in a pyrolysis process to form highly pure boron nitride.This technique is particularly useful for forming high purity boronnitride powders where purity is emphasized over processing throughput.

While description of the feedstock powder and manipulation of theprocessing steps have focused on the provision of a feedstock powderhaving a very fine crystalline size, it is also noted that the feedstockpowder may be fully or partially turbostratic. For example, onembodiment has at least 10% by weight turbostratic content, such as atleast 20, 30, or 40% turbostratic. Certain embodiments may have amajority portion of turbostratic content. In this regard, suchturbostratic boron nitride powder typically has a crystallization indexof less than 0.12. For a more detailed description of the properties andcrystal structure of turbostratic boron nitride, see Hagio et al.,“Microstructural Development with Crystallization of Hexagonal BoronNitride,” J. Mat. Sci. Lett. 16:795–798 (1973), which is herebyincorporated by reference in its entirety.

Typically, the feedstock powder, briquette, log or pellet (all referredto here as compact) has a density within a range of about 0.3 to about2.0 g/cc. In this regard, the compact density of the feedstock powder ismeasured by cutting and weighing a cube of known dimensions on each sidefrom a briquette or log or pellet. Another way of characterizing thefeedstock powder is by measuring initial tap density for furtherprocessing. According to several embodiments of the present invention,the initial tap density is within a range of about 0.2 to about 1.0g/cc.

Referring back to the pressing step 12, typically pressing is carriedout by isostatic pressing, as is well understood in the art. Processingpressures typically exceed 5,000 psi. More typically, greater than about10,000 psi, preferably above 15,000 psi or even 20,000 psi. Followingthe second crushing step 14 and subsequent classification at step 16,generally particles are present that fall outside the scope of thedesired particle size range of the feedstock powder. For example, thecoarse agglomerates that are larger than the target particle sizedistribution may be re-crushed and classified until they are within thetarget size range, while smaller agglomerates and non-agglomeratedparticles that fall below a minimum agglomerate size may be rejectedfrom the feedstock powder. In this latter case, the rejected powder maybe recycled, typically by subjecting the recycled powder to pressingagain at step 12 followed by crushing at step indicated in FIG. 1. Suchrecycled powder is generally mixed with incoming virgin powder formedpursuant to crushing at step 10. Alternatively, pressing step 12 can beaccomplished by uniaxial pressing (pill pressing or tabletting) rollcompacting or briquetting the pressures applied is sufficient to obtaina desired density of the consolidated body, as described previously.Alternatively, compacts can be formed by wet processes, whereby a slurryof boron nitride is spray dried or filtered forming a compact.

While the particle size range for the agglomerated feedstock powder mayvary considerably depending upon the desired end properties in the finalpowder product, typically the particle size of the feedstock powderfalls within a range of about 20 to about 1,000 microns, typically about40 to 500 microns. Narrower particle size ranges within the foregoingbroad ranges are typically processed for tight particle size control inthe final product. As used herein, particle size range is typicallydetermined by the screening technique described above. In this regard,it is noted that screening is not an ideal process, and as such, acertain proportion of undesirable particle sizes may be present, mosttypically, fines may be caught within the product on the bottom screen,thereby shifting the particle size range to be slightly finer thanspecified.

As shown in FIG. 1, following classification, the feedstock powder isthen subjected to a sintering operation at step 18. Here, sintering iseffected to the boron nitride agglomerate in powder form, rather than inany sort of bulk form such as a brick, pellet or log. During thissintering operation, it is typical that agglomerates bond together viaweak inter-agglomerate bonds (necking). Accordingly, it is generallydesirable to subject the heat-treated or sintered powder to a crushingoperation at step 20. As described above in connection with step 10, thecrushing operation at step 20 may be carried out by various techniques,including jaw crushing and coarse roll crushing, although milling istypically not carried out at step 20 so as to preserve as close aspossible the initial particle size (agglomerate) range of the originalfeedstock powder.

Typically, the sintering operation at step 18 is carried out at atemperature to facilitate crystal growth and crystallization of theamorphous phases (turbostratic), so as to form a generally hexagonalcrystal structure in the heat-treated product. In this regard, thesintering temperature is typically greater than at least about 1,400°C., such as within a range of about 1,600 to 2,400° C. Sinteringtemperatures range between 1,800° C. and 2,300° C., and specificsintering temperatures range within about 1,850° C. to about 1,900° C.Typically, the environment during sintering is inert, to minimize orprevent unwanted reactions with the boron nitride feedstock powder. Inthis regard, the sintering furnace is typically evacuated (vacuum) suchas at a pressure less than about 1 atm. Gases present within thesintering environment are typically inert such as argon or nitrogen(N₂). Typical heat-treatment preparations fall within a range of about0.25 to 12 hours, dependent upon furnace design and heating rates.

As a result of the sintering operation, the density of the feedstockpowder is generally reduced, unlike sintering operations with othertypes of ceramic materials. One explanation for the reduction in densityduring“sintering” is that neck formation occurs between adjacentparticles by a non-densifying diffusion process like vapor-phaseTransport. (see Modern Ceramic Engineering, D. W. Richerson, Chapter 7,1982) It is typical to see reduction in density on the order of at least0.1 g/cc, such as at least 0.2 g/cc. Particular examples of heat-treatedpowder exhibited a tap density on the order of about 0.2 g/cc to about1.0 g/cc, following crushing at step 20 and classification at step 22.In this regard, classification at step 22 would be carried out by anyone of the techniques described above in connection with classificationat step 16.

According to an embodiment of the present invention, classification atstep 22 reveals that at least 25 wt. % of the heat-treated boron nitrideagglomerated powder (following crushing) falls within the initialparticle size range of the feedstock powder. Generally the heat-treatedboron nitride powder (following crushing) has an average particle sizeof at least 20 microns, and has a particle size range within about 40microns to about 500 microns. In this regard, it is generally desiredthat the particle size distribution of the final powder productapproximates that of the original feedstock powder. This feature iseffective to improve yield of the final, heat-treated crushed andclassified boron nitride agglomerated powder over state-of-the-artprocessing techniques, such as techniques that rely on heat treatment ofboron nitride in pellet, brick, log or briquette form.

The heat-treated boron nitride agglomerated powder typically has ahexagonal crystal structure. Hexagonal boron nitride is an inert,lubricious ceramic material having a platey hexagonal crystallinestructure (similar to that of graphite) (“h-BN”). The well-knownanisotropic nature of h-BN can be easily explained by referring to FIG.2, which shows hexagons of a h-BN particle. The diameter of the h-BNparticle platelet is the dimension shown as D in FIG. 2, and is referredto as the a-direction. BN is covalently bonded in the plane of thea-direction. The particle thickness is the dimension shown as Lc, whichis perpendicular to diameter and is referred to as the c-direction.Stacked BN hexagons (i.e., in the c-direction) are held together by Vander Waals forces, which are relatively weak.

The final, heat-treated crushed and classified agglomerated boronnitride powder may have a crystal structure that is hexagonal, rangingfrom a highly ordered hexagonal crystal structure to a disorderedhexagonal structure. Such powders typically have a crystallization indexon the order of 0.12 and higher. (See Hubacek, “Hypothetical Model ofTurbostratic Layered Boron Nitride,” J. Cer. Soc. of Japan, 104:695–98(1996), which is hereby incorporated by reference in its entirety.)

In addition, the sintering operation is effective to volatilizeimpurities and surface oxide contaminants. The resulting product, priorto crushing, is a“cake” of weakly aggregated agglomerates that areeasily broken down to a particle size distribution resembling that ofthe initial particle size distribution of the feedstock powder.

It is noted that while recycling steps are shown in FIG. 1 between steps12 and 16, and between steps 20 and 22, recycling steps may be employedbetween various process steps of the basic flow shown in FIG. 1.

The final, agglomerated boron nitride product typically has a relativelyhigh fracture strength, particularly with respect to its envelopedensity (actual agglomerate density) and/or the powder tap density (bulkdensity of the powder). For example, one embodiment has a fracturestrength to tap density ratio of not less than about 11 MPa·cc/g, suchas not less than about 12 MPa·cc/g, 13 MPa·cc/g or even 14 MPa·cc/g. Interms of envelope density, such ratio is typically not less than 6.5MPa·cc/g, such not less than 6.7 MPa·cc/g, or not less than 6.9MPa·cc/g.

As to the powder particle size characterization, the powder may have anaverage agglomerate size within a range of about 20 to 500 μm, such asabout 40 to 200 μm. In this regard, certain embodiments may have atleast 60% of the powder falling within a particle distribution range ofabout 40 to 200 μm, or at least 80% within a range of 40 to 150 μm.

According to embodiments of the present invention, powder havingcharacteristics as described above may be provided. However, otherembodiments utilize the powder in various applications. For example,turning to FIG. 4, a printed circuit board 200 is provided includingmultiple layers 206–218 forming a stack of layers 204. As illustrated,one of the opposite major surfaces includes a plurality of solder bumpsfor electrical interconnection with other electronic components.Although not shown, the opposite major surface of the printed circuitboard may have electrical traces there along for routing electricalsignals. According to the embodiment shown in FIG. 4, any one of ormultiple layers 206–218 may include agglomerates as discussed above.Typically, the loading ratio of agglomerates is sufficient such that theagglomerates provide an interconnected network and generally contacteach other for efficient heat transfer. This interconnected network ofagglomerates is referred to herein as a percolated structure and maygenerally form a skeletal structure that extends through and is imbeddedin a matrix phase. Typically, the matrix phase is formed of a polymer,including organic polymers. In certain embodiments, it is desirable toutilize a thermoplastic polymer for processing ease. Layersincorporating agglomerates according to embodiments of the presentinvention advantageously improve heat transfer of the printed circuitboard for demanding electronic applications.

Turning to FIG. 5, another embodiment of the present invention is shown,including a microelectronic device. In this particular embodiment, themicroelectronic device 300 includes a semiconductor die 300 in apackaged state, namely, flip-chip bonded to an underlying substrate 304,which is provided for electrical interconnection with othermicroelectronic devices. The microelectronic device 300 includes athermal transfer film 310 that includes agglomerates as disclosed above,provided in a matrix phase, typically a polymer such as a resin. Asdiscussed above in connection with the printed circuit board, theagglomerates form an interconnected network or percolated structure forimproving thermal transfer between the integrated circuit 302 and theunderlying substrate 304. As is generally understood in the art,electrical interconnection between the integrated circuit and thesubstrate is achieved through incorporation of solder bumps 306 bondedto respective pads on the semiconductor die, bonded to the substratecontacts through re-flow of the solder material.

According to yet another feature of the present invention, a compositestructural component is provided that includes a matrix phase andagglomerates as discussed above. The composite structural component maytake on various structural forms and in particular embodiments forms anintegral component of a microelectronic device such as a hard diskdrive. In the particular case of the hard disk drive, the component maybe an actuator arm.

Alternatively, the structural component may provide a computer case, asis generally shown in FIG. 6. FIG. 6 illustrates a notebook computer 400having a case having two halves 402 and 404. Case portion 402 generallydefines a structural support for and back surface of an LCD screen ofthe computer, while case portion 404 encloses and protects the sensitivemicroelectronic components of the laptop computer 400 and includes abottom surface 406. The case is desirably formed of a compositematerial, including a matrix phase and agglomerates according toembodiments of the present invention. The matrix phase may be astructurally sound polymer material such as a thermoplastic polymer.While the laptop is shown in FIG. 6, it is understood that the case maybe configured suitable for desktop computers as well as for servers andother computing devices.

Further, among the various types of shells or cases for microelectronicdevices, the structural component may be in the form of a telephonecasing, defining the outer structural surface of a telephone handsetsuch as a mobile telephone.

The following is not an exhaustive list of suitable composite structuralcomponents, and a myriad geometric configurations may be provided forvarious applications, including microelectronic applications. Forexample, the structural component may take on the form of heat pipe asdescribed generally in U.S. Pat. No. 6,585,039, a composite heater as inU.S. Pat. Nos. 6,300,607 and 6,124,579, a brake pad structure as in U.S.Pat No. 5,984,055, or as various overvoltage components as in U.S. Pat.No. 6,251,513.

The following examples reference screening parameters which, unlessotherwise noted, are based on the tensile bolting cloth (TBC) standard.The following Table 1 is provided to convert between TBC mesh size, U.S.Sieve, microns and mils, for ease of interpretation.

TABLE 1 Tensile Bolting Cloth US Sieve TBC Microns Mils 12 1680 66 14 161410 56 16 18 1190 47 18 22 1000 39 20 24 841 33 25 28 707 28 35 38 50020 40 46 420 17 45 52 354 14 50 62 297 12 60 74 250 10 70 84 210 8.3 8094 177 7.0 100 120 149 5.9 120 145 125 4.9 140 165 105 4.1 170 200 863.4 200 230 74 2.9 230 63 2.5 270 53 2.1 325 44 1.7 400 38 1.5

EXAMPLES Example 1

Approximately 50 lbs of a feedstock boron nitride powder comprised offine crystals having a particle size not greater than about 5 μm wasconsolidated with an isostatic press at 20 ksi. The resulting materialwas then crushed using a jaw crusher, then a roll type crusher. Theresulting powder was then screened to separate fine and coarseagglomerates. Coarse agglomerates, for the purpose of this example, wereabove 150 microns and fine agglomerates, below 40 microns. Screening wasconducted utilizing a screener fitted with 120 and 200 TBC (TensileBolting Cloth) screens. The resulting 8 lbs of material that was 60%above 74 microns was heat-treated at approximately 1900° C. for 12 hoursto produce a high purity boron nitride cake. This cake was then crushedusing a jaw crusher, then a roll type crusher. The resulting powder wasthen screened to separate fine and coarse agglomerates. Coarseagglomerates, for the purpose of this example, were above 150 micronsand fine agglomerates, below 150 microns. Agglomerates greater than 150microns in size were re-crushed until they were within the targetagglomerate size range, i.e. less than 150 microns, for this example.The resulting 2 lbs of screened material was 95% between 150 microns and74 microns and possessed a tap density of approximately 0.50 g/cc. Thestrength of selected particles, 125 microns in diameter, was 8.2 MPa.

Example 2

Approximately 50 lbs of a feedstock boron nitride powder comprised offine crystals having a particle size not greater than about 5 μm wascrushed using a jaw crusher, then a roll type crusher. The resultingpowder was then screened to separate fine and coarse agglomerates.Coarse agglomerates, for the purpose of this example, were above 150microns and fine agglomerates, below 40 microns. Screening was conductedutilizing a screener fitted with 120 and 200 TBC (Tensile Bolting Cloth)screens. The resulting 5 lbs of material that was 60% above 74 micronswas heat-treated at approximately 1900° C. for 12 hours to produce ahigh purity boron nitride cake. This cake was then crushed using a jawcrusher, then a roll type crusher. The resulting powder was thenscreened to separate fine and coarse agglomerates. Coarse agglomerates,for the purpose of this example, were above 150 microns and fineagglomerates, below 150 microns. Agglomerates greater than 150 micronsin size were re-crushed until they were within the target agglomeratesize range, i.e. less than 150 microns, for this example. The resulting2 lbs of screened material was 95% between 150 microns and 74 micronsand possessed a tap density of approximately 0.35 g/cc. The strength ofselected particles, 125 microns in diameter, was 4.5 MPa.

Example 3

Approximately 100 lbs of a feedstock boron nitride powder comprised offine crystals having a particle size not greater than about 5 μm wasconsolidated with an isostatic press at 20 ksi. The resulting materialwas then crushed using a jaw crusher, then a roll type crusher. Theresulting powder was then screened to separate fine and coarseagglomerates. Coarse agglomerates, for the purpose of this example, wereabove 200 microns and fine agglomerates, below 40 microns. Agglomeratesgreater than 200 microns in size were recrushed until they were withinthe target agglomerate size range, i.e. less than 200 microns, for thisexample. The fine agglomerates and crystallites that were producedduring crushing, typically below 10 microns in size, were separated fromthe larger agglomerates by air classification. The resulting 18 lbs. ofcoarse product was screened utilizing a screener fitted with a 88 and120 TBC (Tensile Bolting Cloth) screens (Kason Corporation, Millburn,N.J.). The resulting 3 lbs of material that was 60% above 150 micronswere heat-treated at approximately 1900° C. for 12 hours to produce ahigh purity boron nitride cake. This cake was then crushed using a jawcrusher, then a roll type crusher. The resulting powder was thenscreened to separate fine and coarse agglomerates. Coarse agglomerates,for the purpose of this example, were above 200 microns and fineagglomerates, below 200 microns. The resulting 2 lbs of screenedmaterial was 95% between 200 microns and 74 microns and possessed a tapdensity of approximately 0.50 g/cc. The strength of selected particles,150 microns in diameter, was 7.5 MPa.

Following the process flows above, additional samples were prepared,having a range of tap densities, provided below in Table 2 as Examples5–7. In addition, Comparative Examples 1, 2, and 3 were prepared forcomparative testing. The comparative examples were prepared in a mannersimilar to Examples 1–3 described above, with a significant differencethat the comparative examples were created based upon heat treating thematerial in log or briquette form, rather than agglomerated powder formas described above in connection with embodiments of the presentinvention. All samples were subjected to agglomerate strength testing,by utilizing the testing apparatus shown in FIG. 3. The apparatusconsists of a small load frame 100 with a movable stage 110. The stagemotion was in x, y, and z direction and was controlled by steppingmotors (Newport PM 500 Precision Motion Controller). The z-motion wasset to 2 μm/s. A 4.9 N (500 g) load cell 102 was used and the sampleswere placed on highly polished and parallel SiC anvils 104, 106. Thestage motion was controlled through LabView. Data acquisition wasperformed at a sampling rate of 20 datapoints/s resulting in aresolution of 0.1 μm and 0.01 N.

Samples for testing were prepared by hand-picking agglomerates ofsimilar size from the sample batches, and single agglomerates weretested for fracture strength. The effective tensile strength of eachagglomerate was calculated assuming the irregular shape is somewherebetween a sphere and a cube, yielding

$\sigma_{e} = {1.37\frac{P}{a^{2}}}$

where P is the fracture load and α the diameter (size) of theagglomerate.

Tap density was measured in accordance with ASTM B527-70.

Envelope density was measured by Hg porosimetry, by infiltrating Hgliquid under a pressure of 40 Kpsi. Envelope density represents theaverage density of the agglomerates of the sample, as opposed the tapdensity, which is a bulk density measurement.

TABLE 2 Strength/ρ Strength/ρ Fracture Tap (tap) Envelope (envelope)strength Density Agglomerate ratio Density ratio ID [MPa] (g/cc) Size[μm] [MPa · cc/g] (g/cc) [MPa · cc/g] CE1 5.1 .48 125 10.6 .807 6.32 E18.2 .48 125 17.1 .792 10.38 E2 4.5 .35 125 12.9 .648 6.94 CE2 5.7 .68125 8.4 1.078 5.29 CE3 4.4 .71 125 6.2 1.396 3.15 E3 7.5 .48 150 15.6.755 9.93

According to the foregoing, relatively high strength, controlled densitypowders are provided, and in particular, high strength, low and mediumdensity powders are provided for that are particularly suitable forthermally conductive applications. In addition, the powders may havegenerally isotropic behavior, thermally and structurally. Still further,embodiments of the present invention also provide processing techniquesfor forming agglomerated boron nitride powders. Such processingtechniques are highly flexible, and are effective to improve yield, andaccordingly, be cost effective. While powders according to embodimentsof the present invention may be particularly used for thermal conductionapplications, such as in the semiconductor art, the processingtechniques are flexible, capable of creating boron nitride powders forother applications.

1. A boron nitride agglomerated powder comprising: an agglomeratefracture strength to tap density ratio not less than about 11 MPa·cc/g.2. The powder of claim 1, wherein the ratio is not less than about 12MPa·cc/g.
 3. The powder of claim 1, wherein the ratio is not less thanabout 13 MPa·cc/g.
 4. The powder of claim 1, wherein the ratio is notless than about 14 MPa·cc/g.
 5. The powder of claim 1, wherein thepowder has an average agglomerate size within a range of about 20 to1000 μm.
 6. The powder of claim 1, wherein the powder has an averageagglomerate size within a range of about 40 to 500 μm.
 7. The powder ofclaim 1, wherein the powder has an average agglomerate size within arange of about 40 to 200 μm.
 8. The powder of claim 1, wherein thepowder has an average agglomerate size within a range of about 20 to 90μm.
 9. The powder of claim 1, wherein at least 60% by weight of thepowder is within a particle distribution range of about 40 to 200 μm.10. The powder of claim 1, wherein at least 80% by weight of the powderis within a particle distribution range of about 40 to 150 μm.
 11. Aboron nitride agglomerated powder comprising: a fracture strength toenvelope density ratio not less than about 6.5 MPa·cc/g.
 12. The powderof claim 11, wherein the ratio is not less than about 6.7 MPa·cc/g. 13.The powder of claim 11, wherein the ratio is not less than about 6.9MPa·cc/g.
 14. A method for forming a boron nitride agglomerated powder,comprising: providing a feedstock powder comprising boron nitrideagglomerates, the feedstock powder comprising fine crystals having aparticle size not greater than about 5 μm; and heat treating thefeedstock powder to form a heat treated boron nitride agglomeratedpowder.
 15. The method of claim 14, wherein the feedstock powder has anaverage crystal size, the average crystal size being not greater than 5μm.
 16. The method of claim 14, wherein the average crystal size is notgreater than 2 μm.
 17. The method of claim 14, further comprising:exposing the heat treated boron nitride agglomerated powder to acrushing operation.
 18. The method of claim 17, wherein the feedstockpowder has an initial particle size range, and at least 25 wt % of theheat treated boron nitride powder following crushing falls within theinitial particle size range.
 19. The method of claim 17, wherein afterthe crushing operation, the heat treated boron nitride powder isclassified by particle size.
 20. The method of claim 19, wherein afterclassification, the heat treated boron nitride powder has an averageparticle size of at least 20 μm.
 21. The method of claim 19, whereinafter classification, the heat treated boron nitride powder has aparticle size distribution within a range of about 20 μm to about 1000μm.
 22. The method of claim 14, wherein the heat treated boron nitridepowder has a hexagonal crystal structure.
 23. The method of claim 14,wherein the feedstock powder comprises a turbostratic crystal structure.24. The method of claim 14, further comprising a step of classifying abulk boron nitride agglomerated powder by particle size, wherein thefeedstock powder is a portion of the bulk boron nitride agglomeratedpowder having a desired particle size range.
 25. The method of claim 24,wherein the feedstock powder that falls within the desired particle sizerange is less than the entirety of the bulk boron nitride agglomeratedpowder, and a remainder of the bulk boron nitride agglomerated powder isrecycled.
 26. The method of claim 14, wherein the feedstock powder isformed by: crushing a solid boron nitride body to form a bulk boronnitride agglomerated powder; and removing a desired particle size rangeof the bulk boron nitride agglomerated powder to form the feedstockpowder.
 27. The method of claim 14, wherein the step of heat treating iscarried out at a temperature of at least 1400° C.
 28. The method ofclaim 27, wherein the temperature falls within a range of about 1600° C.to 2400° C.
 29. A method for forming a boron nitride agglomeratedpowder, comprising: providing a bulk boron nitride powder containingboron nitride agglomerates; removing a portion of the boron nitrideagglomerates to form a feedstock powder comprising boron nitrideagglomerates; and heat treating the feedstock powder to form a heattreated boron nitride agglomerated powder.
 30. The method of claim 29,wherein the portion removed corresponds to a desired particle sizerange.
 31. The method of claim 29, wherein removal of the portion of theboron nitride agglomerates is executed by classifying the bulk boronnitride powder by particle size.
 32. A method for forming a boronnitride agglomerated powder, comprising: providing a feedstock powdercomprising boron nitride agglomerates, the feedstock powder comprisingturbostratic boron nitride; and heat treating the feedstock powder toform a heat treated boron nitride agglomerated powder.
 33. The method ofclaim 32, wherein the feedstock powder comprises at least 10 wt %turbostratic boron nitride.